Methods are being developed for the fluorescent double staining of DNA and histones, and these are being applied to the study of the relationship between the two substances during the cell cycle, and during cell differentiation. Analysis is done by flow-cytometery. Model systems currently being employed include Ehrlich ascites cells, rat spermatogenic cells and reticuloendothelial virus infected chicken bone marrow cultured cells. The staining method currently applicable for double staining and dual parameter analysis are propidium iodide for DNA and eosin Y for histone. Fluorescent antibodies against nucleosomal histones are being prepared as a second histone staining reagent.